Diurnal Wind Variations Research Articles

Natural Infrasound as an Atmospheric Probe

Summary For four years continuous recording of infrasonic signals in the frequency range 0.1 to 1 Hz, known as microbaroms, has been conducted at Palisades, New York. The microbaroms we recorded are radiated into the atmosphere by interfering ocean waves in the North Atlantic as far as 2000 km away. A characteristic diurnal variation in the amplitude of the received signal has been noted, independent of any variation in the source. We conclude that the variation is due to variations of the factors affecting atmospheric sound propagation, namely wind and temperature. In winter a semidiurnal variation in signal amplitude is observed, with maximum reception around 11 : 00 and 22; 00 local time. Reference to wind and temperature observations in the literature shows that at these times the lowest level of reflection of the vertically propagating signal occurs between 100 and 110 km due to the presence of strong east winds. At 18 : 00, time of minimum amplitudes, the reflection level rises to about 115 km because of a change in tidal wind phase. Viscous dissipation associated with the changed reflection height can account for the observed signal weakening. A third maximum, a less regular effect, is found to be related to more variable winds between 95 and 105 km. In summer, reflection is found to occur from about 50 km due to the presence of stratospheric easterlies. The summer diurnal variation, different from that of the winter, exhibits only a weak minimum about 20 : 00. This appears to result from a diurnal temperature variation superimposed on a diurnal wind variation. Abnormally high microbaroms were recorded at times that can be related to an atmospheric event known as a stratospheric warming. Microbaroms thus provide a continuously available natural mechanism for probing the upper atmosphere. We conclude that the establishment of microbarom observation systems could give a comprehensive technique for monitoring several upper atmospheric parameters.

A planetary‐scale land–sea breeze circulation in East Asia and the western North Pacific

AbstractThe diurnal wind variation over the East Asian continent is commonly considered to be a combination of a land‐sea breeze near the coast and a mountain–valley breeze along the slopes of the Tibetan Plateau. The local land–sea breeze along the coastline typically spans < 100 km into the ocean. However, a detailed examination of the global reanalysis data suggests that this local land–sea breeze circulation apparently couples with the global‐scale diurnal atmospheric pressure tide to produce a planetary‐scale land–sea breeze with a spatial scale of ∼1000 km over the western North Pacific. Computations of the momentum budget and equivalent potential temperatures indicate that the atmospheric diurnal tidal wave contributes the most to this circulation feature. A diagnosis of the water vapour budget further suggests that the convergence of water vapour flux, which is related to the convergence of low‐level wind induced by the seasonal change of diurnal tidal wave, leads to different times of occurrence of maximum diurnal rainfall over East Asia between summer and winter. Copyright © 2010 Royal Meteorological Society

Annual and Diurnal Wind Variations Originating from Japanese Islands

The convergent and divergent wind over the Japanese islands is estimated using meteorological observation data to examine the general characteristics of land and sea breeze driven by differential heating of land and sea. The results show an annual cycle wind called the “Japanese Monsoon” blows, as well as a diurnal cycle flow. These winds are caused by the temperature difference between the Japanese islands and the surrounding sea. The divergent wind field over the Japanese islands can be expressed simply as a superposition of synoptic-scale phenomena (wind fields with seasonal and daily cycles). An annual variation in the amplitude of the diurnal cycle wind is closely related to the magnitude of solar radiation.

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

Diurnal variation of surface wind over central eastern China

Hourly wind observations from 452 meteorological stations are used to document the diurnal cycle of the surface wind over the central eastern China (100°–122°E, 20°–42.5°N). Both the surface wind speed and the wind direction show large diurnal variation with pronounced topographic effects. At most stations, the surface wind speed reaches the maximum in the afternoon and the minimum in early-morning. This diurnal phase shows small seasonal variation, whereas the diurnal amplitude varies significantly in different seasons. The diurnal amplitude of the surface wind speed reaches maximum in spring over the northern and southwestern China and in summer over the southern China. The diurnal cycle of the wind direction is more complicated. Over the coastal (mountain) regions, the diurnal wind direction is greatly influenced by the land–sea (mountain–valley) breezes with large (small) seasonal variation. Over the northern plain region, the wind direction exhibits small diurnal variation but with remarkable seasonal rotation. The surface wind over the stations located on the top of mountains shows distinct diurnal variation, which represents the diurnal cycle of the tropospheric low-level wind. The wind speed over these stations is highest in pre-dawn and lowest in the afternoon. The wind anomaly rotates clockwise from late night to late afternoon, and shows significant seasonal variation as influenced by the annual cycle of the monsoon system. The contribution of the diurnal surface wind to the diurnal feature of precipitation is briefly discussed.

Diurnal Variation of Downslope Winds in Owens Valley during the Sierra Rotor Experiment

The impact of diurnal forcing on a downslope wind event that occurred in Owens Valley in California during the Sierra Rotors Project (SRP) in the spring of 2004 has been examined based on observational analysis and diagnosis of numerical simulations. The observations indicate that while the upstream flow was characterized by persistent westerlies at and above the mountaintop level the cross-valley winds in Owens Valley exhibited strong diurnal variation. The numerical simulations using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) capture many of the observed salient features and indicate that the in-valley flow evolved among three states during a diurnal cycle. Before sunrise, moderate downslope winds were confined to the western slope of Owens Valley (shallow penetration state). Surface heating after sunrise weakened the downslope winds and mountain waves and eventually led to the decoupling of the well-mixed valley air from the westerlies aloft around local noon (decoupled state). The westerlies plunged into the valley in the afternoon and propagated across the valley floor (in-valley westerly state). After sunset, the westerlies within the valley retreated toward the western slope, where the downslope winds persisted throughout the night.

A Climatology of Diurnal and Semidiurnal Surface Wind Variations over the Tropical Pacific Ocean Based on the Tropical Atmosphere Ocean Moored Buoy Array

Abstract Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E–95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for the meridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (∼0.14 m s−1) and phase [maximum westerly wind anomalies ∼0325/1525 local time (LT)] across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator (maximum southerly wind anomalies ∼0700 LT at 5°N and ∼1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (June–November). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ∼0700 LT (opposite conditions ∼1900 LT); the amplitude of the divergence diurnal cycle is ∼5 × 10−7 s−1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force.

The Formation of Nocturnal Rainfall Offshore from Convection over Western Kalimantan (Borneo) Island

This study investigates the diurnal variation of convection over western Kalimantan (Borneo) Island and the adjacent seas through analysis of results from surface meteorological observations, satellite observations of convection and precipitation, and numerical experiments with a mesoscale model. Precipitation measurements from the TRMM satellite indicated that rainfall occurs mostly in the afternoon and evening along the coast of the island, and at night and in the early morning over the plains in the central regions of the island and over the sea adjacent to the coast. Results from surface meteorological observations showed that intense shortwave radiation is observed on the western coast at Siantan on most days, even on days with heavy rain. Consequently, sea breezes are evident in the afternoon, whereas land breezes prevail in the early morning. Occasionally, a strong offshore flow is observed by the QuikSCAT satellite over the sea adjacent to the western coast of the island in the early morning, with calm surface winds on the coastal land.Climate modeling studies for a 1-month period during the region’s rainy season successfully replicated the main features of the observed regional distribution, diurnal variation of surface winds and convection over the western coast of the island and the adjacent seas. The results showed that convections develop across a wide area along the coast of the island at similar times in the late afternoon with the penetration of the sea breeze. The convections cause a rapid drop in temperature in the lower atmosphere, which produces a temperature contrast between the land and the sea, with lower temperatures occurring over the island. In response to the temperature differences, a strong offshore flow occurs over the sea adjacent to the western coast during nighttime, which creates an intensive wind convergence at the low-levels, and initiates convection offshore near the coast at late night and early morning. The results of this study suggest that the strong offshore flow derived principally from convections that develop earlier in the afternoon and evening over the island plays an important role in the formation of the nocturnal rainfall over the sea in the vicinity of western Kalimantan Island.

Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system

In the present communication, initial results from the allSKy interferometric METeor (SKiYMET) radar installed at Thumba (8.5°N, 77°E) are presented. The meteor radar system provides hourly zonal and meridional winds in the mesosphere lower thermosphere (MLT) region. The meteor radar measured zonal and meridional winds are compared with nearby MF radar at Tirunalveli (8.7°N, 77.8°E). The present study provided an opportunity to compare the winds measured by the two different techniques, namely, interferometry and spaced antenna drift methods. Simultaneous wind measurements for a total number of 273 days during September 2004 to May 2005 are compared. The comparison showed a very good agreement between these two techniques in the height region 82–90 km and poor agreement above this height region. In general, the zonal winds compare very well as compared to the meridional winds. The observed discrepancies in the wind comparison above 90 km are discussed in the light of existing limitations of both the radars. The detailed analysis revealed the consistency of the measured winds by both the techniques. However, the discrepancies are observed at higher altitudes and are attributed to the contamination of MF radar neutral wind measurements with Equatorial Electro Jet (EEJ) induced inospheric drifts rather than the limitations of the spaced antenna technique. The comparison of diurnal variation of zonal winds above 90 km measured by both the radars is in reasonably good agreement in the absence of EEJ (during local nighttime). It is also been noted that the difference in the zonal wind measurements by both the radars is directly related to the strength of EEJ, which is a noteworthy result from the present study.

Diurnal variations of temperature and winds inferred from TIMED and UARS measurements

Owing to their sampling properties in local solar time, both the TIMED and UARS spacecrafts provide the opportunity to quantitatively study the mesospheric thermal tides. The information though is limited because it takes the TIMED instruments 2 months to cover the full range of 24 hours, almost twice as long as UARS. We present here the results of an analysis, in which the seasonal and global‐scale variations of amplitude and phase for the migrating diurnal tide are retrieved from atmospheric temperature and wind measurements on the TIMED (SABER, TIDI) and UARS (MLS, HRDI) satellites. Comparisons are made with results from the Numerical Spectral Model (NSM), which is characterized in part by the parameterization of small‐scale gravity waves. At 60 km and above, the amplitude and phase variations of the diurnal tide from the NSM agree generally well with those derived from the SABER temperatures. In the SABER temperature amplitudes at around 60 km there are evident asymmetries in latitude with respect to the equator, which are not apparent in the MLS data near 55 km or in the NSM. (The asymmetry in the SABER temperatures disappears above 60 km.) For the meridional winds of the migrating diurnal tide derived from TIDI, the phase and relative amplitude variation with latitude and season are in qualitative agreement with the HRDI measurements and the NSM. The amplitudes from TIDI winds are systematically smaller, but this may in part be due to interannual variations. However, the results for the zonal winds from TIDI more often do not agree well with those from HRDI or the NSM.

Raindrop size distribution observed with the Equatorial Atmosphere Radar (EAR) during the Coupling Processes in the Equatorial Atmosphere (CPEA‐I) observation campaign

The diurnal variability of raindrop size distribution (DSD) in precipitating clouds over Kototabang, West Sumatra, Indonesia (0.20°S, 100.32°E), is studied using three types of Doppler radars, operated at VHF (47 MHz), UHF (1.3 GHz), and X band (9.4 GHz) frequencies. Two precipitation events from 5 to 6 May 2004 in the first observation campaign of the first Coupling Processes in the Equatorial Atmosphere (CPEA‐I) project reveal a difference between clouds precipitating in the early afternoon and clouds precipitating in the nighttime. In the early afternoon, the precipitating clouds were dominated by shallow convective types with high rainfall rate at the surface. In the nighttime, precipitating clouds were dominated by stratiform types with small rainfall rate at the surface. A diurnal variation of horizontal wind was observed over this area. The westerly in the lower troposphere and the easterly in the middle troposphere began to be enhanced in the afternoon (1400–1700 LT). DSD parameters were retrieved from VHF band Doppler radar data. A modified gamma distribution was used to model DSD parameters. The shape parameter (μ) was larger during stratiform precipitation than during shallow convective precipitation events, as shown by previous studies. During stratiform rain events on 5 May 2004, the median volume diameter (D0) was dominantly greater than 1 mm, which is larger than D0 during shallow convective rain events. Results presented in this paper indicate that DSD has a diurnal cycle over the mountainous region of Sumatra.

Nocturnal jet and its effects on early morning rainfall peak over northeastern Bangladesh during the summer monsoon season

Diurnal variations of rainfall and upper wind over Bangladesh during the summer monsoon season were investigated through the data analysis of raingauge, 4‐times‐daily rawinsonde and pilot balloon observations. Over the northeastern part where the total monsoon rainfall is heavy a rainfall peak appears at 3 to 6 local time in the early morning. In the lower troposphere, the southeasterly accelerates in the evening, and the wind direction exhibits a clockwise change at night. This feature corresponds with the nocturnal jet, which can be a cause of the midnight‐early morning rainfall peak in the northeastern part of Bangladesh through the increased wind blowing against the southern edge of the Meghalaya Mountains.

Seasonal and Interannual Variations of Diurnal Cycles of Wind and Cloud Activity Observed at Serpong, West Jawa, Indonesia

In this paper, the seasonal changes in the diurnal variations of wind and the cloud activity at Serpong (106.7°E, 6.4°S), near Jakarta, are climatologically described. In the dry season (May-October), diurnal variation of wind accompanied with sea-land breeze circulation was prominent. In the rainy season (November-April), the diurnal variation was consistent with sea-land breeze circulation, but was not as clear as that in the dry season. The peak time of the northerly in the rainy season, similarly to that of the sea breezes at Serpong in the low level (below 1.0 km height), was earlier than that in the dry season. The maximum time in the climatological diurnal variation of the surface temperature at Serpong in the rainy season was earlier than that in the dry season. The vertical structure which was consistent with sea breeze circulation was clearer when the prevailing (daily-mean) wind was weaker in the rainy season. These results are consistent with the features of the local circulation; in other words, the local circulation depends on the diurnal variation of surface temperature and is prominent when the prevailing wind is weak. The typical diurnal variation of the wind in the rainy season was unclear when the prevailing northwesterly to westerly was strong around Serpong. Interannual variation of the diurnal variation could be detected in the transitional period from the dry to the rainy season. Cloud activity had prominent diurnal variation over West Jawa in the rainy season and was active in the early evening over land, particularly, in the mountainous area in the south of Serpong. When cloud activity was active over the mountainous area, the northerly below 1.0 km in height was prominent at Serpong, which is consistent with the feature that the development of a local cloud system is accompanied with local circulation.

Diurnal, semiannual, and solar cycle variations of solar wind–magnetosphere–ionosphere coupling

Using the Km index, the F10.7 index, and solar wind data from 1965 to 1996, we have examined the variations for the efficiency of solar wind–magnetosphere–ionosphere (SMI) coupling. It is found that the efficiency of SMI coupling depends on the total solar zenith angle, which is defined by the sum of cosine of the solar zenith angles at the northern and southern corrected geomagnetic (CGM) poles. This result suggests that the diurnal and semiannual variations of the coupling efficiency are explained by the variations of the total solar zenith angle. Further, we find that the coupling efficiency during the period of high solar activity (F10.7) tends to be lower than that during the period of low solar activity. This suggests that the efficiency of SMI coupling has a dependence on the solar activity. Our interpretation of these characteristics is that the total Pedersen conductivity on both northern and southern polar caps controls the efficiency of SMI coupling. We find that the variations of the coupling efficiency can be reproduced when our empirical conductivity model is applied in the Siscoe‐Hill model. This suggests that the ionospheric conductivity plays a significant role in SMI coupling and produces diurnal, semiannual, and solar cycle variations of geomagnetic activity.

Regional accuracy of QuikSCAT gridded winds

The quality of gridded 00 UTC and 12 UTC QuikSCAT wind speed fields provided by the Florida State University (FSU) and NASA Jet Propulsion Laboratory (JPL) are analysed over the Bay of Bengal during May–August 2001. Additionally, an examination of these fields is performed over the Gulf of Mexico for the May–August period from 2001 to 2003. Both 00 UTC and 12 UTC time almost coincide with QuikSCAT sampling times (twice a day) and correspond to either early morning or late evening local time over these regions. The primary restriction for generating accurate maps with a temporal resolution of 12 hours and less is a lack of adequate sampling. Due to non‐uniform spatial‐temporal sampling of the scatterometer, interpolation procedures cannot provide proper estimates in data gaps over the regions not covered by a swath. Wind speed autocorrelation coefficients for gridded datasets have been compared with that of original QuikSCAT data and buoy winds. It is shown that the spatial and temporal interpolation used to obtain these datasets results in smoothing of the QuikSCAT wind speeds, reducing their original variance. This smoothing is amplified where substantial diurnal wind variability occurs. A comparison with buoy data shows that FSU and JPL gridded fields are unable to reproduce correctly observed low correlations in wind speed between morning and evening breeze flows and diurnal variability of winds, leading to a reduced difference between 00 UTC and 12 UTC values in comparison with buoys and swath QuikSCAT data. Rather, the FSU and JPL maps describe daily mean fields. Another consequence of the spatial‐temporal interpolation is that the winds are distorted at a frequency matching the dominant sampling interval (3–4 days) of QuikSCAT measurements over the Bay of Bengal.

The diurnal variation of the marine surface wind in an offshore flow

AbstractData from a number of light vessels moored off the English coast have been used to show that the marine near‐surface wind within a distance of 40 km to 80 km from the coast varies diurnally when the wind is blowing in an offshore direction and the sea is cooler than the maximum overland air temperature. This variation is such that the maximum wind speed is overnight while the minimum wind speed is during the afternoon. This is in distinction to overland surface winds where the diurnal maximum in the wind strength occurs during the afternoon. A numerical model of the stable boundary layer has been used to simulate the variation, and the mechanisms involved are compared with those operating in stable layers over the land. This gives evidence that the main reason for the variation is the difference in boundary‐layer temperature profiles between the daytime and night‐time offshore flows. During the day the stable layer formed advectively over the sea reduces downward momentum transfer and hence the surface wind. During the night the stable layer preformed over the land evolves during advection to form an elevated inversion over the sea which, for reasons discussed, causes the overnight maximum. As well as having implications for maritime forecasting, model results show that vertical wind shears in the stable layers affect the performance of sailing vessels. © Crown copyright, 2005. Royal Meteorological Society

Global observations of the land breeze

Four‐times daily satellite wind observations from the QuikSCAT and ADEOS‐II tandem scatterometer mission are used to study the land/sea breeze circulation. These observations provide a global view of diurnal wind variations over the ocean. Results agree with frictional linear theory: winds follow an elliptical hodograph through the course of the day, but indicate that diurnal wind variations propagate offshore progressively like nonlinear gravity waves and are detectable several hundred kilometers from the coast.

Wind-driven lateral variability in a partially mixed estuary

A circulation study of the upper Neuse River Estuary (NRE) was initiated to characterize the nature of motion in the lateral dimension and to understand the role that this motion may play in the occurrences of warm weather fish kills. Low frequency wind-driven variability in across channel bottom salinity was identified and attributed to variability in the synoptic wind band. Diurnal variability in bottom salinity appeared to be driven by diurnal wind variability, presumably associated with the local sea breeze. In both cases the lateral response of the system was driven predominantly by across channel wind forcing and not by Ekman setup associated with along channel winds. Additional variability at near diurnal periods may have been due to baroclinic seiches. A lateral, barotropic seiche with a period of ∼30 min was also identified. High salinity water was found to be a good marker for low DO water across the estuary. Wind-driven upwelling of low DO/high salinity water and downwelling of high DO/low salinity water was documented and provides a plausible causal mechanism for the death of pelagic fish in the upper NRE.

Annual and Diurnal Wind Patterns in the City of São Paulo

The major topographic, mesoscale, and urban influences on the wind patterns of the City of Sao Paulo are characterized using one year of surface wind velocity data observed at 11 surface stations within its urban limits. The data was used to study the diurnal and annual variations of wind velocity and horizontal wind divergence within the city. Results showed that the circulation over the investigated area is dominated by three major factors: sea breeze; mountain-valley circulations; and urban effects, such as roughness, building-barrier, and urban heat island. The sea breeze was found to be the dominant feature of the monthly-averaged diurnal variation of Sao Paulo surface winds during the eight warmest months of the year. The sea breeze front induces a velocity minimum at the time of its passage and a post-frontal afternoon velocity maximum. Mountain-valley thermal effects on the flow can be seen in the temporal divergence/convergence patterns. These thermal effects tend to be more important during colder months, at night, and when the wind velocities are low. Nighttime downslope convergent flows are present over the city during winter and spring and daytime upslope divergent flows are present over the city during summer months.

The diurnal wind variation in a variable eddy viscosity semi-geostrophic Ekman boundary-layer model: Analytical study

¶A time-dependent semi-geostrophic Ekman boundary-layer model (SG), including slowly varying eddy diffusivity with height and inertial term effects, is developed to investigate the diurnal wind variation in the planetary boundary layer (PBL). An approximate analytical solution of this model is derived by using the WKB method, which extends the Tan and Farahani (1998)’s solution by including the vertical variable eddy viscosity. The features of the diurnal wind variation in the PBL mainly depend on three factors: the latitude, horizontal momentum advection and eddy viscosity. The vertical variable eddy viscosity has little influence on diurnal wind variation in the PBL at the low latitude, however its effect may be exacerbated in the mid- and high latitudes. In comparing with the constant eddy viscosity case, the decreasing (increasing) with height eddy viscosity produces a large (small) maximum wind speed (MWS) in the PBL, however, the eddy viscosity that has a mid-layer peak in the vertical gives rise to a higher height of occurrence of MWS. For the boundary-layer wind structure, there is a singular point when the modified SG inertial oscillation frequency η equals the forcing frequency ω. The isotachs of boundary-layer wind speed have almost no tilt to left or right relative to time evolution and the occurrence time of the MWS is the earliest at the singular point. The feature will be enhanced in the decreasing with height eddy viscosity case and weakened in the eddy viscosity initially increasing with height case.